Device for tracing seams for welding by electron bombardment

ABSTRACT

This invention concerns electron bombardment welding techniques. The invention teaches a method and a device for keeping a welding beam centered on a seam plane. The detector includes two detectors arranged symetrically with respect to a seam plane. They furnish a signal whose amplitude increases as the electronic beam spot moves away from the seam plane center and approaches the edge of one of the pieces being joined. A servo-system, without oscillation, brings the beam back into the axis of the seam plane. The device can be used in welding big pieces.

Corcelle et al. July 3, 1973 [54] DEVICE FOR TRACING SEAMS, FOR3,426,174 2/1969 Graham et al 219/121 EM WELDING BY ELECTRON BOMBARDMENT3,329,813 4/1967 Hashimoto 250/495 3,513,285 5/1970 lmura 219/121 EM 1Inventors: Francois C l 70 is u u 3,472,997 10/1969 El-Kareh et a1219/121 EB Docteur Oge, Montlhery; Jean 3,152,238 10/1964 Anderson219/121 EB Angely, 153 bis Avenuetie la 3,276,902 10/1966 Abraham219/121 EM X Republique, Villejuif; Jean-Pierre Delort, 21 rue de laCondamine, Primary Examiner-J. V. Truhe Paris, all of France AssistantExaminer-Gale R. Peterson [22] Filed: May 5 1971 Attorney-Sughrue,Rothwell, Mion, Zinn & Macpeak [21] Appl. N0.: 140,399 57 ABSTRACT Thisinvention concerns electron bombardment weldgn Application Priority Dataing techniques. The invention teaches a method and a May 5, 1970 France7016360 device for p g a Welding beam Centered On a Seam plane. Thedetector includes two detectors arranged [52] US. Cl. 219/121 EB ym t lywith respect to a seam p n h y ur ish [51] Int. Cl 323k 15/00 a ig awhose amplitude increases as the r ni [58] Field of Search 219/121 R,121 EM, beam p moves y from the Seam P center and 219/121 EB; 250/495 R,49,5 A approaches the edge of one of the pieces being joined. Aservo-system, without oscillation, brings the beam 56] Referen es Cmdback into the axis of the seam plane. The device can be UNITED used inwelding pieces. 3,609,288 9/1971 Sciaky 219/121 EM 6 Claims, 3 DrawingFigures Patented July 3,1913 I 3,743,776

2 Shoots-Sheet 1 FIG .1

Patented July 3, 1973 3,743,776

2 Shoots-Shoot l BACKGROUND OF THE INVENTION The invention pertains to amethod and a device which makes it possible, during the course ofelectron bombardment welding, to keep the electron beam on the seambetween two pieces to be welded together, without manual action.

The electron welding beam spot must be kept on the seam with goodprecision if we want to benefit from the advantages derived from thestrong penetration power of such an electron beam. This beam placementis relatively easy when the position of the pieces to be welded togetheris defined with sufficient precision with respect to the beam and whenthe welding joint has a simple geometric shape. But this is not the casewhen the seam line has just any shape.

Most of the earlier devices make it possible to perform precisionwelding only by using very precise mechanical systems. With suchdevices, it is notpossible to weld big boiler pieces, because of theweight of these pieces and because of the imprecision with which theirforms are known.

A more reliable device has been perfected. This device is described inFrench Pat. No. 1,589,437, filed on May 6, 1968. This device employs ascreen whose edge has exactly the same shape as the welding line'to bemade. The edge of this screen is placed parallel to the line which thespot must follow, at a small distance 3 from the spot. By deflecting thebeam with respect to the welding line, we can lead the spot to the edgeof this screen. We can thus verify its position which we then correct ifnecessary.

An even more recent device consists in beforehand chamfering the sameinclination of the edges of the two pieces to be welded and to use theelectrons, which are back scattered by these chamfers, in order to keepthe beam on the welding line. Another device causes the beam line to bepalpated in advance by a beam with reduced power and thus corrects thedirection of the beam little by little.

Although these devices offer a partial solution to the problem, theynevertheless require preparatory work which causes loss of time andmoney and which also introduces a certain impression in the position ofthe beam.

SUMMARY OF THE INVENTION This invention makes it possible to remedythese inthe electron beam with respect to the plane of the scam in thecourse of welding two pieces of any shape together.

The invention employs the back scattered electrons of the incident beam.It is based on the following finding: So long as the beam is kept in theaxis of the plane of the seam, back scattering is weak. The moment thebeam moves very slightly away from its position and comes to hitdirectly on a surface next to the seam plane in the course of welding,the back scattering intensity grows abruptly in a dissymmetricalfashion.

The method involved in this invention concerns the maintenance of thewelding electronic beam in the plane of the seam of the two pieces to bewelded together by using the measurement of the flow of primary backscattered electrons and is characterized by the conveniences and toguarantee optimum positioning of fact that an increase in the flow ofback scattered electrons along the edge of one of the pieces controls atransverse displacement of the beam and brings it back into the seamplane at its optimum position.

The device which employs the above method includes two back scatteredelectron pick up probes, arranged in a symmetrical fashion on eitherside of the seam plane. These probes are connected to an electronic andsafety servo-system and are further characterized by the fact that theyare so oriented that they will exclusively pick up the flow of electronsback scattered by the plane of the seam and the edge of the two piecesto be welded together.

The device described above offers the advantage of using probes having asimple circular cylindrical form and is characterized by the fact thatthese probes are equipped, at their inputs, with a screen in which anopening has been made, the diameter of this opening being adapted to theintensity of the incident welding beam. Further, the probes arecharacterized by the fact that self-polarization keeps them at apotential that eliminates charges other than the true back scatteredelectrons.

We thus get rapid, repetitive and precise measurements. In order toexploit them completely, it is necessary to use a servo-system whichwill enable us very rapidly to return the beam to the center of the seamline without producing a transverse oscillatory move ment which isparticularly bad in a welding operation.

The device described above thus includes a servosystem which keeps thebeam in the center of the seam line and which returns it there withoutoscillation in the case of deviation from the welding line. It ischaracterized by the fact that the probes are connected through adifferential amplifier to a digital/analog converter with N stages, theoutput of which is a potential proportional to the displacement of thebeam spot from the center of the seam plane. The output from thedigitallanalog converter is applied to a differential amplifier, thenceapplying on the deflection coils a signal whose amplitude dependsdirectly on the difference between the voltage furnished by thedifferential amplifier to the digital/analog converter and a pre-setvoltage, so that the amplitude is cancelled out when this differencebecomes zero.

Finally, to make the device reliable, it is necessary to provide for thecase in which the spot, having left the seam plane, would have atendency to move away abnormally, for example, as a result of thebreakage of an element in the circuits employed in the adjustment of theposition of the beam.

The device according to the invention thus involves a safety systemcharacterized by the fact that it consists of two gates with pre-setthresholds, acting by any known device, in order to warn the operator toreduce the power of the beam when the signal has reached an anticipatedthreshold value and that the spot is moving away from the welding planeby a pre-determined quantity.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of theinvention will emerge from the following description of the inventionwhenread with the accompanying drawings wherein:

FIG. 1 illustrates the basic circuit diagram of a device for carryingout the teachings of the invention,

FIG. 2 illustrates the details of the servo-system of the invention, and

FIG. 3 is a graph of the currents sensed by the probes as a function ofthe spot position.

- DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With respect to FIG.1, there is shown at 1 and 2 two pieces to be welded together,represented schematically by two plates which are joined up to 4 by awelding seam 3. The nonwelded parts of pieces 1 and 2 are separated by avery narrow slot 5. An electronic beam 6 is furnished by a gun 7 and itsbeam concentration device is symbolized by a coil 8. The deflectionsystem 9 can divert the beam 6. On either side of the line of the seam,we have probes l and 11, in the shape of diaphragmequipped cylinderswith openings, such as 12 and 13, adapted to the power of the beam 6.These probes are connected to a servo-system 14, equipped, at the input,with a differential amplifier l5, acting upon the deflecting system 9.Finally, we note the safety line 16 which controls the beamconcentration deviceJWhen the seam line of the two pieces is straightand when the beam is correctly regulated, the welding operation goes onwithout any trouble; the two identical probes, which are placedsymmetrically with respect to the plane of the seam, collect equalcharges if the openings 12 and 13 are equal and if the two probes are atthe same distance from the pieces in the course of welding. If, for anundetermined reason, the beam 6 leaves the plane of the seam or if thewelding line ceases to be straight, one of the two detectors, theright-hand detector, for example, will collect a bigger charge and as aresult a signal will be transmitted to the servo-system 14, through thedifferential amplifier 15. This servo-system will deliver a signal tothe deflection coil 9 so as to displace the beam 6 toward the left andbring it back to 4.

It goes without saying that the flow of back scattered electronsundergoes very rapid increase when the spot progressively leaves thewelding line. The moment we have the slightest deviation of the spot, wethus get a signal sufficient to bring about the action of theservosystem 14.

In order to prevent inhibiting the probes, it was necessary to adapt, atthe input of these probes, a diaphragm whose opening is adapted to thewelding power used so as to furnish a sufficient signal the moment thebeam has left the center of the seam plane and to continue to furnish asignal with a not excessive value when the spot ceases to be partiallyabsorbed by the seam plane.

A system for the self-polarization of the probes prevents them fromcollecting the thermal electrons and the ions of all kinds which arefound abundantly above a welding line during the time the beam isfunctioning. This self-polarization can be obtained very simply byputting a resistance 40, 41 in series with the conductor coming out ofthe probe before the differential amplifier 15 of the servo-system 14.After the initial moment, the current, which flows through thisresistance, polarizes the probe with respect to the ground. We regulatethe value of this resistance so that the probe will pick up only thetrue back scattered electrons.

FIG. 2 illustrates in greater detail the servo-system of the presentinvention.

The conductors coming out of probes 10 and 11 lead to the differentialamplifier 15 which is placed at the input of the servo-system. Theoutput ofthe differential amplifier 15 is connected to two thresholdgates 17 and 18. The gates control, respectively, the logic gates 19 and20. These gates are connected on either side to a digital/analogconverter 21 with N steps.

Element 21 is responsive to pulses from pulse source 37 and produces ananalog output proportional to the displacement of the beam spot from thecenter of the seam plane. Thus, it can be seen that element 21 functionsas digital to analog converter and will hereinafter be so identified.

The digital/analog converter acts upon the differential amplifier 22,one of whose inputs 23 is kept at a potential equal to one-half thevoltage which the converter would deliver for N steps. This differentialamplifier in turn acts upon the deflection coils 9 of the electron gun.

When the electron beam centered upon the seam plane moves ever soslightly away from the center of the seam plane causing a portion of thespot to be back scattered by the flat surface of one of the pieces inthe course of welding, there is an increase in the signal transmitted bythe probe, which is on the side toward which the spot has moved. Thedifferential amplifier 15 then transmits a signal which will be positiveor netative, depending upon whether it comes from probe 10 or 11. Thesize of this signal depends on the intensity of the back scattered flow,and hence, on the quantity by which the spot has moved away from thecenter of the seam joint. When this magnitude is greater than a value 6determined in advance, the threshold gate 18 transmits the signal to thelogic gate 20. If, on the other hand, the signal was negative, it wouldthen correspond to information picked up by probe 11, and the thresholdgate 17 would allow the signal to pass if the latter were negative andif it has an absolute value greater than 6. The latter, in turn, wouldthen open the logic gate 19.

The logic gate 20 starts the counting in the digitallanalog converter 21when the voltage, coming from the differential amplifier 15, ispositive, while as the logic gate 29 starts the deduction in thedigital/analog converter 21 when the voltage coming from thedifferential amplifier is negative. The digital/analog converter 21 withN steps is placed, in the initial state, in

position N/2, in such a manner that its output voltage will be equal tohalf of the voltage V which it would deliver for N steps. The otherinput to the differential amplifier 22 is a fixed voltage equal to thevoltage V IZ so that, if the beam is correctly directed after theinitial moment, there is no action upon the deflection coils, so long asthe voltage delivered by the digital/analog converter 21 has not varied,in terms of absolute value, by a value greater than V /N.

If the displacement of the beam involves an action of the differentialamplifier 15 and then one of the threshold gates 17 or 18 and finallythe opening of one of the gates 19 or 20, the digital/analog converter21 delivers a voltage V which is different from the voltage V /Z.

its output, a signal to the coils 9 which brings about a displacement ofthe spot toward the left, in the assumption visualized here. Theelectron beam thus tends to come back to the welding line. Itprogressively ceases to be retrodiffused in a dissymmetrical fashion andthe amplifier retains its last value. The beam has then come back to itsbalanced position.

In FIG. 2, we can also see a safety device involving two threshold gates23 and 24, adjusted for voltages corresponding to the signal which theprobes would furnish if the beam were to deviate in a noteworthyfashion. In this case, one of the threshold gates 23 or 24 would actupon a device 25, which is known in itself, which modifies the voltageapplied to the electron gun and reduces the power of the beam while atthe same time it gives rise to a signal intended for the operator.

FIG. 3 gives us a better understanding of the operation of theservo-device and the safety system by showing a graphic image.

For a certain adjustment of probes 11 and 12, we have shown, inarbitrary units, the current furnished at the output of the differentialamplifier when the spot occupies the positions 31, 32, 33, 34, 35, and36 with respect to the edges 26 and 27 of the two pieces to be weldedtogether.

We can see that the current is zero when the spot occupies the position34 exactly on the median line of the seam plane and also touches the twoedges of the two pieces.

We also note that the moment the spot clearly hits the edge 26 or 27 ofthe pieces 1 and 2 which are to be welded together, the current passesthrough a maximum. It is thus possible to regulate the safety thresholdgates 23 and 24 in keeping with values corresponding to the positions 33and 35 and thus to prevent the beam, at full power, from attacking thepositions 32 or 31 or 36, which would damage the pieces to be welded.

Finally we must note that the method does not require the initialprecise positioning of the beam on the welding seam. The device involvedin this invention is in effect capable of detecting the seam withsufficient precision so that the servo-syytem begins to operate and thenitself places the beam in the optimum welding position.

What is claimed is:

l. A device for keeping an electronic welding beam, operating with itsfull intensity, centered in a seam plane comprising:

an electron gun, including deflection coils for producing said weldingbeam,

a pair of detectors disposed symmetrically about said seam plane formeasuring the flow of primary back scattered electrons resulting fromsaid welding beam, the flow of primary back scattered electrons receivedby one of the detectors abruptly increasing as the welding beamintersects an edge of one of the pieces being welded along the seamplane, means, coupled to said detectors, for producing a signalindicative of the difference in the flow of said primary back scatteredelectrons measured by said probes and means responsive to said signalfor controlling said deflection coils to center said welding beam in theseam plane in response to the compared value of said flow.

2. The device of claim I, wherein said means for producing saiddifference indicative signal includes first differential amplifier meanscoupled to said probes, said means for controlling including digital toanalog converter means for producing a potential proportional to theoutput of said differential amplifier means, means for generating apre-set voltage, and second differential amplifier means, one input ofsaid second dit ferential amplifier means being coupled to said pre-setvoltage generating means, another input of said second differentialamplifier means being connected to the output of said digital to analogconverter means, the output of said second differential amplifier meansbeing coupled to said deflection coils.

3. The device of claim 2, further including resistance means seriallyconnected between said probes and said first differential amplifiermeans for blocking thermal electrons and ions for entering the probeswhereby said probes receive only back scattered electrons.

4. The device of claim 3, wherein said probes are cylindrical in shape,the inputs thereto including a screen having an aperture, the diameterthereof being caused to vary in accordance with the intensity of theincident welding beam.

5. The device of claim 4, wherein said means for controlling furtherincludes a safety system comprising: threshold gates means coupled tosaid first differential amplifier means, an operator signal and controlmeans coupled to said threshold gate means for generating and operatorsignal and decreasing theelectron beam power in response to an outputfrom said threshold gate means indicating that the beam spot hasdeviated from the seam plane by a predeten'nined amount.

6. The device of claim 2 further including; a pulse source for alteringthe digital content of said digital to analog converter, a pair ofcoincident gates receiving pulses from said pulse source and coupledrespectively to an incrementing and decrementing input of saidconverter, threshold gate means coupled between said first differentialamplifier and said pair of coincident gates for selectively enabling oneof said gates in response to the polarity of a signal having at least apredetermined value from said first differential amplifier.

1. A device for keeping an electronic welding beam, operating with itsfull intensity, centered in a seam plane comprising: an electron gun,including deflection coils for producing said welding beam, a pair ofdetectors disposed symmetrically about said seam plane for measuring theflow of primary back scattered electrons resulting from said weldingbeam, the flow of primary back scattered electrons received by one ofthe detectors abruptly increasing as the welding beam intersects an edgeof one of the pieces being welded along the seam plane, means, coupledto said detectors, for producing a signal indicative of the differencein the flow of said primary back scattered electrons measured by saidprobes and means responsive to said signal for controlling saiddeflection coils to center said welding beam in the seam plane inresponse to the compared value of said flow.
 2. The device of claim 1,wherein said means for producing said difference indicative signalincludes first differential amplifier means coupled to said probes, saidmeans for controlling including digital to analog converter means forproducing a potential proportional to the output of said differentialamplifier means, means for generating a pre-set voltage, and seconddifferential amplifier means, one input of said second differentialamplifier means being coupled to said pre-set voltage generating means,another input of said second differential amplifier means beingconnected to the output of said digital to analog converter means, theoutput of said second differential amplifier means being coupled to saiddeflection coils.
 3. The device of claim 2, further including resistancemeans serially connected between said probes and said first differentialamplifier means for blocking thermal electrons and ions for entering theprobes whereby said probes receive only back scattered electrons.
 4. Thedevice of claim 3, wherein said probes are cylindrical in shape, theinputs thereto including a screen having an aperture, the diameterthereof being caused to vary in accordance with the intensity of theincident welding beam.
 5. The device of claim 4, wherein said means forcontrolling further includes a safety system comprising: threshold gatesmeans coupled to said first differential amplifier means, an operatorsignal and control means coupled to said threshold gate means forgenerating and operator signal and decreasing the electron beam power inresponse to an output from said threshold gate means indicating that thebeam spot has deviated from the seam plane by a predetermined amount. 6.The device of claim 2 further including; a pulse source for altering thedigital content of said digital to analog converter, a pair ofcoincident gates receiving pulses from said pulse source and coupledrespectively to an incrementing and decrementing input of saidconverter, threshold gate means coupled between said first differentialamplifier and said pair of coincident gates for selectively enabling oneof said gates in response to the polarity of a signal having at least apredetermined value from said first differential amplifier.